The present invention relates to a magnetic recording medium which comprises a substrate and a magnetic recording layer composed of a coating composition containing a binder resin and magnetic particles and, more particularly, to a magnetic recording medium which is excellent in the orientation property and the packing property of the magnetic particles in a coating film (magnetic recording layer) due to the excellent dispersibility of the magnetic particles in a vehicle, and is produced by the manufacturing process in which the amount of dispersant used can be reduced, or in some cases, no dispersant is necessary.
With progressing miniaturization, weight reduction and recordable-time prolongation of video or audio magnetic recording and reproducing devices in recent years, magnetic recording media such as a magnetic tape and a magnetic disk have been strongly required to have a higher performance, namely, to record at a higher recording density and to lower the noise level.
Especially, since the demands for video tapes having a higher picture quality has been increasingly strong, the frequencies of carrier signals recorded are higher than in a conventional video tape. In other words, the carrier signals recorded are shifted to a short-wavelength region and, as a result, the magnetization depth from the surface of a magnetic tape becomes greatly small.
Accordingly, the efforts to keep and further improve the high output characteristics, in particular, the CN ratio of short-wavelength signals have been made. For this purpose, it is necessary (1) to produce very fine magnetic particles, (2) to make the surface of the magnetic coating film smooth, (3) to make the magnetic coating film thin, and (4) to improve the dispersibility of the magnetic particles.
This fact is described in, for example, DEVELOPMENT OF MAGNETIC MATERIALS AND TECHNIQUE OF IMPROVING THE DISPERSION PROPERTY OF MAGNETIC POWDER (1982), published by K. K. Sogo Gijutsu Kaihatsu Center, p. 312: "The condition for high-density recording in a coating type tape is that it is possible to keep the high output characteristics with respect to a short-wavelength signal at a low noise level. For this purpose, it is necessary that both the coercive force (Hc) and the residual magnetism (Br) are large, and that the thickness of the coating film is thin"; and Nikkei Electronics (1976), May 3, pp. 82 to 105: "As the physical quantities related to the SN ratio (CN ratio) of a luminance signal, there are average number of particles per unit volume, the dispersing state (dispersibility) and the smoothness of the surface thereof. If the smoothness and the dispersibility are constant, since the SN ratio becomes higher in proportion to the root of the average number of particles, the smaller volume and the higher packing density magnetic particles have, the more advantageous."
As described above, in order to improve the high output characteristics with respect to short-wavelength signals, not only are magnetic particles required to have a high dispersibility but also the magnetic coating film is required to be thin. When the magnetic coating film is made thin, it is essential to improve the packing property of the magnetic particles in order to keep or improve the other properties. For this purpose, it is necessary to reduce the amount of component other than the magnetic particles, in particular, dispersant, lubricant, etc. as much as possible, or in some cases, to dispense with the need of any such component. Thus, the development of a binder having an excellent resin adsorptivity to the magnetic particles is strongly demanded.
On the other hand, it is known that the noise level of a magnetic recording medium has a tendency to be lowered as the particle size of the magnetic particles used becomes smaller, namely, the specific surface area becomes larger, as shown, for example, in FIG. 3 on page 27, lines 23 to 29 in the REPORTS OF TECHNICAL RESEARCHES OF THE INSTITUTES OF ELECTRONICS AND COMMUNICATION ENGINEERS OF JAPAN MR 81-11. Recently, the particle size of magnetic particles has a tendency to be increasingly reduced. In this report, FIG. 3 shows the relationship between the particle size of Co-coated acicular maghemite particles and the noise level, and it is clear from FIG. 3 that as the particle size becomes small (the specific surface area of the particles becomes large), the noise level lowers linearly. It is known, however, that as the particle size of the magnetic particles becomes small, the dispersibility of the magnetic particles in a vehicle is lowered, resulting in the deterioration of the orientation property and the packing property in the coating film.
As a method of improving the dispersibility of magnetic particles in a vehicle, methods of adding various kinds of dispersants, of using various dispersing machines, of improving various properties of the magnetic particles, and of improving various properties of a binder resin (varying the kinds of a binder resin) are known.
Especially, the kinds of a binder resin is an important factor for influencing the dispersion of magnetic particles in a vehicle. This fact is described in DEVELOPMENT OF MAGNETIC MATERIALS AND TECHNIQUE OF IMPROVING THE DISPERSION PROPERTY OF MAGNETIC POWDER, p. 65: "The influence of a binder on the dispersion of magnetic powder is large. For example, even if the dispersing machine used has a high dispersion efficiency, in the case where the dispersibility of the binder is low, the magnetic particles in the coating do not disperse . . . The dispersibility is greatly different depending upon the kinds of a binder."
As a conventional binder resin, nitrocellulose, a polyvinylidene chloride, a vinyl chloride-vinyl acetate copolymer, a styrene-butadien copolymer, a polymethyl methacrylate, a vinylidene chloride-methyl methacrylate copolymer, a polyurethane and an epoxy resin are generally used.
As a magnetic recording medium with the magnetic particles having an improved dispersibility in a vehicle, a magnetic recording medium having a magnetic recording layer which contains fine magnetic particles uniformly dispersed in a polyester which has a reducing viscosity of not less than 0.2 and is produced by using a dicarboxylic acid having a sulfonic acid metal base as a copolymerizing component in the ratio of 0.3 to 7.0 mol % based on the total carboxylic acid component, is exemplified [Japanese Patent Application Laid-Open (KOKAI) No. 54-28603 (1979)].
The strongest demand in the manufacture of a magnetic recording medium is the improvement of the dispersibility of magnetic particles in a vehicle and the increase in the resin content adsorbed to the magnetic particles. Even if the polyester produced by using as a copolymerizing component a dicarboxylic acid containing a sulfonic acid metal base [described in Japanese Patent Application Laid-Open (KOKAI) No. 54-28603 (1979)] is used, the dispersibility of the magnetic particles in a vehicle cannot be said satisfactory and the resin adsorption is small. Furthermore, it is pointed out that the method of improving the dispersibility of the magnetic particles in a vehicle by adding a large amount of dispersant is disadvantageous in that it is impossible to improve the packing property of the magnetic particles and in that the dispersant remaining in the coating film (magnetic recording layer) influences the surface property of the coating layer such as the viscosity, which may lead to defective operation due to a change in the ambient temperature.
Therefore, the offer of a magnetic recording medium and a method of producing the same which allows the amount of dispersant used in the manufacturing process to be reduced as much as possible, or in some cases, dispenses with the need of any dispersant due to an excellent dispersibility of magnetic particles in a vehicle and the excellent packing property of the magnetic particles in the coating film (magnetic recording layer) which is brought about by the large resin adsorption of the magnetic particles, is strongly demanded.
As a result of the present inventors' studies on a magnetic recording medium which allows the amount of dispersant used in the manufacturing process to be reduced as much as possible, or in some cases, dispenses with the need of any dispersant, it has been found that when a substrate is coated with a coating composition containing a polyester having an iminosulfonyl group as a binder and magnetic particles so as to form a magnetic recording layer thereon, the dispersibility of the magnetic particles in a vehicle is very excellent and the packing property of the magnetic particles in the resultant coating film (magnetic recording layer) is very excellent. The present invention has been achieved on the basis of this finding.